A persistent sodium current contributes to oscillatory activity in heart interneurons of the medicinal leech

Abstract1.Normal activity in bilateral pairs of heart interneurons, from ganglia 3 or 4, in the medicinal leech (Hirudo medicinalis) is antiphasic due to their reciprocally inhibitory connections. However, Ca+-free Co+-containing salines lead to synchronous oscillations in these neurons.2.Internal TEA+ allows expression of full plateaus during Co++ induced oscillations in heart interneurons; these plateaus are not blocked by Cs+. Similar plateaus are also observed with internal TEA+ alone, but under these conditions activity in heart interneurons from ganglia 3 or 4 is antiphasic.3.Plateaus in heart interneurons induced by Co++ and internal TEA+ involve a conductance increase.4.A voltage-dependent inward current, IP, showing little inactivation, was isolated using single-electrode voltageclamp in heart interneurons. This current is carried at least in part by Na+; the current is reduced when external Na+ is reduced and is carried by Li+ when substituted for Na+.5.Calcium channel blockers such as La3+ and Co++ block neither the TEA+ induced plateaus nor IP, suggesting that Na+ is not using Ca++ channels. Moreover, IP is enhanced by Ca++-free Co++-containing salines. Thus, IP is correlated with the TEA+- and Co++-induced plateau behavior.